1. Field of the Invention
The presently disclosed invention relates to apparatus and methods for shaping transparencies and, more particularly, apparatus and methods for shaping one or more transparency plies in a heating process.
2. Discussion of the Prior Art
Vehicle transparencies such as windshields and windscreens, are usually laminated structures that include two layers (i.e. “plies”) of glass that are bound together by a thermoplastic material, such as vinyl. Flat sheets of glass (e.g., float glass) that are larger than the desired size of the laminated structure are cut to size to create inner and outer glass plies. The edges of the glass plies are ground, the plies are washed, and a ceramic paint is often applied to a portion or portions of one or both of the plies. The plies are heated and shaped, either one ply at a time (i.e., a singlet process) or at the same time with both plies stacked (i.e., a doublet process). The shaping process is accomplished by pressing the plies into the final form using a press tool. Generally, a lower press ring lifts the plies and presses them on to a top press to achieve the desired shape and dimensional characteristics. A thermoplastic material layer or layers are inserted between the plies and the laminated structure is heated in an autoclave such that the desired clarity and visual characteristics of the laminated structure are achieved.
Vehicle manufacturers often design vehicle glass panels that utilize glass having curves of smaller radii to improve wiperability, reduce weight, improve the vehicle's aerodynamic properties, lower the vehicle's profile, etc. In forming such curves, the forming process must be designed such that the glass is not overstressed to the point that the glass breaks or buckles, thus creating optical and reflective distortion (i.e., a lens effect). However, current manufacturing techniques often result in such deleterious effects. When a lower ring pushes the glass from the outside perimeter over the press shape, stresses are created in the glass that cause localized buckling around the perimeter of the glass.
In the prior art, glass manufacturers attempted to minimize the effect of such bucking by including an additional step in the manufacturing process. Generally, the glass was pressed into shape according to the conventional process of forming the center of the glass first and then forming the perimeter areas of the glass. This process resulted in wrinkles and buckles in the glass and caused optical distortions. Counter forces were then applied to the formed glass to alleviate the wrinkles, buckles and other distortions resulting from this process. However, such buckling is a function of several variables including glass thickness, depth of the required bend in the glass, and the time that the glass is allowed to bend during the forming process. Such complexities made the use of counter forces to alleviate folds, wrinkles, buckles and the like difficult and produced somewhat unpredictable results. Accordingly, such prior art systems and methods have resulted in slower manufacturing processes and higher breakage rates making the overall process more costly. Moreover, such processes still failed to produce transparencies of acceptable quality for some applications.
For example, newer model vehicles offer optional features that demand a higher degree of fidelity to the precise surface contours and dimensions of the windshield design. For example, some vehicles offer a feature for automatic activation of the breaking system in response to road obstructions. In another example, some current model vehicles offer a feature by which the firmness of the vehicle suspension system is regulated to anticipate and counteract irregularities in the road surface. Such systems are controlled by comparison of road images that are taken simultaneously from different, forward looking vantage points from inside the vehicle. The image comparison is provided to the breaking control system or the suspension control system. The brakes and the suspension are adjusted according to the size and location of the sensed obstruction or irregularity. The forward looking images are captured by cameras that view the roadway through the windshield. Both systems require the cameras to provide images of relatively high resolution within a relatively short response time. For vehicles with such breaking and suspension features, distortions in the windshield of a degree that previously may have been acceptable are unacceptable for those portions of the windshield that are in the field of view of the camera. Such distortions may cause the braking system or the suspension control system to misinterpret road conditions and cause the system to fail to react or to react improperly. Accordingly, there was a need in the prior art for systems and methods for improving the surface quality and dimensional control of windshields—especially those windshields on vehicles equipped with high-performance braking and suspension options such as those described herein. Thus, there was a need for glass panel manufacturing processes and equipment that allows for glass panels to be formed while minimizing the negative effects of bending the glass and for improving the surface quality and dimensional control of transparencies.